The present invention relates to ferromagnetic thin-film structures exhibiting relatively large magnetoresistive characteristics and, more particularly, to such structures used for the storage and retrieval of digital data.
Many kinds of electronic systems make use of magnetic devices including both digital systems, such as memories, and analog systems such as magnetic field sensors. Digital data memories are used extensively in digital systems of many kinds including computers and computer systems components, and digital signal processing systems. Such memories can be advantageously based on the storage of digital symbols as alternative states of magnetization in magnetic materials provided in each memory storage cell, the result being memories which use less electrical power and do not lose information upon removals of such electrical power.
Such memory cells, and magnetic field sensors also, can often be advantageously fabricated using ferromagnetic thin-film materials, and are often based on magnetoresistive sensing of magnetic states, or magnetic conditions, therein. Such devices may be provided on a surface of a monolithic integrated circuit to provide convenient electrical interconnections between the device and the operating circuitry therefor.
Ferromagnetic thin-film memory cells, for instance, can be made very small and packed very closely together to achieve a significant density of information storage, particularly when so provided on the surface of a monolithic integrated circuit. In this situation, the magnetic environment can become quite complex with fields in any one memory cell affecting the film portions in neighboring memory cells. Also, small ferromagnetic film portions in a memory cell can lead to substantial demagnetization fields which can cause instabilities in the magnetization state desired in such a cell.
These magnetic effects between neighbors in an array of closely packed ferromagnetic thin-film memory cells can be ameliorated to a considerable extent by providing a memory cell based on an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory thin-film is provided. Such an arrangement provides significant “flux closure,” i.e. a more closely confined magnetic flux path, to thereby confine the magnetic field arising in the cell to affecting primarily just that cell. This result is considerably enhanced by choosing the separating material in the ferromagnetic thin-film memory cells to each be sufficiently thin. Similar “sandwich” structures are also used in magnetic sensors.
In the recent past, reducing the thicknesses of the ferromagnetic thin-films and the intermediate layers in extended “sandwich” structures, and adding possibly alternating ones of such films and layers, i.e. superlattices, have been shown to lead to a “giant magnetoresistive effect” being present in some circumstances. This effect yields a magnetoresistive response which can be in the range of up to an order of magnitude or more greater than that due to the well known anisotropic magnetoresistive response.
In the ordinary anisotropic magnetoresistive response, varying the difference occurring between the direction of the magnetization vector in a ferromagnetic thin-film and the direction of sensing currents passed through that film leads to varying effective electrical resistance in the film in the direction of the current. The maximum electrical resistance occurs when the magnetization vector in the field and the current direction therein are parallel to one another, while the minimum resistance occurs when they are perpendicular to one another. The total electrical resistance in such a magnetoresistive ferromagnetic film can be shown to be given by a constant value, representing the minimum resistance, plus an additional value depending on the angle between the current direction in the film and the magnetization vector therein. This additional resistance has a magnitude characteristic that follows the square of the cosine of that angle.
Operating magnetic fields imposed externally can be used to vary the angle of the magnetization vector in such a film portion with respect to the easy axis of that film. Such an axis comes about in the film because of an anisotropy therein typically resulting from depositing the film during fabrication in the presence of an external magnetic field oriented in the plane of the film along the direction desired for the easy axis in the resulting film. During subsequent operation of the device having this resulting film, such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the film magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the film's easy axis. The state of the magnetization vector in such a film can be measured, or sensed, by the change in resistance encountered by current directed through this film portion. This arrangement has provided the basis for a ferromagnetic, magnetoresistive anisotropic thin-film to serve as a memory cell.
In contrast to this arrangement, the resistance in the plane of a ferromagnetic thin-film is isotropic for the giant magnetoresistive effect rather than depending on the direction of the sensing current therethrough as for the anisotropic magnetoresistive effect. The giant magnetoresistive effect involves a change in the electrical resistance of the structure thought to come about from the passage of conduction electrons between the ferromagnetic layers in the “sandwich” structure, or superlattice structure, through the separating nonmagnetic layers with the resulting scattering occurring at the layer interfaces, and in the ferromagnetic layers, being dependent on the electron spins. The magnetization dependant component of the resistance in connection with this effect varies as the sine of the absolute value of half the angle between the magnetization vectors in the ferromagnetic thin-films provided on either side of an intermediate nonmagnetic layer. The electrical resistance in the giant magnetoresistance effect through the “sandwich” or superlattice structure is lower if the magnetizations in the separated ferromagnetic thin-films are parallel and oriented in the same direction than it is if these magnetizations are antiparallel, i.e. oriented in opposing or partially opposing directions. Further, the anisotropic magnetoresistive effect in very thin films is considerably reduced from the bulk values therefor in thicker films due to surface scattering, whereas a significant giant magnetoresistive effect is obtained only in very thin films. Nevertheless, the anisotropic magnetoresistive effect remains present in the films used in giant magnetoresistive effect structures.
A memory cell based on the “giant magnetoresistive effect” can be provided by having one of the ferromagnetic layers in the “sandwich” construction being prevented from switching the magnetization direction therein from pointing along the easy axis therein in one to the opposite direction in the presence of suitable externally applied magnetic fields while permitting the remaining ferromagnetic layer to be free to do so in the same externally applied fields. In one such arrangement, a “spin-valve” structure is formed by providing an antiferromagnetic layer on the ferromagnetic layer that is to be prevented from switching in the externally applied fields to “pin” its magnetization direction in a selected direction. In an alternative arrangement often termed a “pseudo-spin valve” structure, the ferromagnetic layer that is to be prevented from switching in the externally applied fields is made sufficiently thicker than the free ferromagnetic layer so that it does not switch in those external fields provided to switch the free layer.
Thus, a digital data memory cell based on the use of structures exhibiting the giant magnetoresistive effect is attractive as compared to structures based on use of an anisotropic magnetoresistive effect because of the larger signals obtainable in information retrieval operations with respect to such cells. Such larger magnitude signals are easier to detect without error in the presence of noise thereby leading to less critical requirements on the retrieval operation circuitry.
An alternative digital data bit storage and retrieval memory cell suited for fabrication with submicron dimensions can be fabricated that provides rapid retrievals of bit data stored therein and low power dissipation by substituting an electrical insulator for a conductor in the nonmagnetic layer. This memory cell can be fabricated using ferromagnetic thin-film materials of similar or different kinds in each of the magnetic memory films used in a “sandwich” structure on either side of an intermediate nonmagnetic layer which ferromagnetic films may be composite films, but this intermediate nonmagnetic layer conducts electrical current therethrough based primarily on a quantum electrodynamic effect “tunneling” current.
This “tunneling” current has a magnitude dependence on the angle between the magnetization vectors in each of the ferromagnetic layers on either side of the intermediate layer due to the transmission barrier provided by this intermediate layer depending on the degree of matching of the spin polarizations of the electrons tunneling therethrough with the spin polarizations of the conduction electrons in the ferromagnetic layers which are set by their magnetization directions to provide a “magnetic valve effect”. Such an effect results in an effective resistance or conductance characterizing this intermediate layer with respect to the “tunneling” current therethrough. In addition, shape anisotropy is used in such a cell to provide different magnetization switching thresholds in the two ferromagnetic layers by forming one of the ferromagnetic layers to be thicker than the other. Such devices may be provided on a surface of a monolithic integrated circuit to thereby allow providing convenient electrical connections between each such memory cell device and the operating circuitry therefor.
A “sandwich” structure for such a memory cell, based on having an intermediate thin layer of a nonmagnetic, dielectric separating material with two major surfaces on each of which a anisotropic ferromagnetic thin-film is positioned, exhibits the “magnetic valve effect” if the materials for the ferromagnetic thin-films and the intermediate layers are properly selected and have sufficiently small thicknesses. The resulting “magnetic valve effect” can yield a response which can be several times in magnitude greater than that due to the “giant magnetoresistive effect” in a similar sized cell structure.
As stated above, operating magnetic fields imposed externally can be used to vary the angle of the magnetization vector with respect to the easy axis in the ferromagnetic films of these various kinds of memory cell devices. Such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the film magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the film's easy axis, the state of the cell determining the value of the binary bit being stored therein. One of the difficulties in such memories is the need to provide memory cells therein that have extremely uniform switching thresholds and adequate resistance to unavoidable interjected magnetic field disturbances in the typical memory cell state selection scheme used. This scheme is based on selective externally imposed magnetic fields provided by selectively directing electrical currents over or through sequences of such cells so that selection of a cell occurs through coincident presences of such fields at that cell.
Such a coincident interjected magnetic fields memory cell state selection scheme is very desirable in that an individual switch, such as that provided by a transistor, is not needed for every memory cell, but the limitations this selection mode imposes on the uniformity of switching thresholds for each memory cell in a memory make the production of high yields difficult. FIG. 1 is a diagram representing such a data storage selection scheme having memory cells, represented by rectangle-like structures with triangle-like structures on the left and right ends thereof, having crossing current conductors, represented by lines (including the dashed line portions over the cells in the horizontal lines), forming a grid of such conductors arranged to provide two conductors crossing in the vicinity of each cell (or, in the alternative, the horizontal conductors may be in portions connected together by the cells so that the horizontal dashed lines over the cells are ignored). Thus, the crossing conductors for storing data in the cells may be parallel sense lines each connecting the cells along it in series with one another with the remaining lines perpendicular thereto being word lines each merely passing near to the cells along it or, alternatively, both perpendicular sets of lines for writing may be word lines each passing near to the cells along it (with a further set of sense lines connecting the cells in rows in series with one another not shown also being provided). The horizontal conductors n−1, n, n+1 can have corresponding ones of electrical currents . . . Ixn−1, Ixn, Ixn+1 . . . established in them by selective switching, and vertical conductors m−1, m, m+1 can have corresponding ones of electrical currents . . . Iym−1, Iym, Iym+1 . . . established in them by selective switching.
In operation, each cell, when not to be selected for a state imposition therein, has no currents in the pair of conductors crossing adjacent to them or has current in only one of that pair, either of the Ix or Iy corresponding currents. A selected cell, on the other hand, coincidently receives both of the Ix and Iy corresponding currents along the two perpendicular lines crossing by it in FIG. 1, and is thereby switched into the desired memory state by the coincident magnetic fields generated by each of these currents. These currents, however, must be selected so that neither of the Iy or Ix corresponding currents by itself disturbs the memory state of any of the cells along the conductor carrying that current because all of the others store currently wanted data. Memory cells along a conductor carrying a current Ix or memory cells along a conductor carrying a current Iy that are not being written are subjected to such “half-select” currents, either of the Ix or Iy corresponding currents, which will contribute magnetic fields which increase the chances of combining with other unwanted magnetic fields that on occasion may be present to thereby disturb the states of those memory cells, and so the data stored therein.
If very large currents are used to insure the writing of the highest switching threshold cells, then the half-select currents are correspondingly large and tend to increase the chances of memory cell state disturbances in the lowest switching threshold, or most disturbance sensitive, cells. Thus, the states of the half-selected memory cells are not nearly as stable in storing the bit values therein, and they provide the majority of projected cell data storage failures over time. In addition to half-select currents, these cells must withstand stray magnetic fields from neighboring cells, fields from leakage currents and stray environmental fields. Hence, there is a need to have the switching thresholds of the highest switching threshold cells not be too different from the switching thresholds of the lowest switching threshold cells, and so the resulting requirements for switching threshold uniformity and small switching current design margins tend to make memories implementing the coincident interjected magnetic fields memory cell state selection scheme difficult to manufacture.
The various magnetoresistive memory cell types mentioned above also use a coincident interjected magnetic fields memory cell state selection scheme selection scheme for retrieving stored data. The original MRAM concept cell, based on the anisotropic magnetoresistive effect, and the pseudo-spin valve concept cell, based on the “giant magnetoresistive effect”, both use magnetic such selection schemes for data retrievals, which introduce further data disturbance opportunities through again generating magnetic fields that can combine with unwanted magnetic fields that appear. Magnetic tunnel junction memories often use a diode or transistor to select a memory cell for data retrievals, and thus do not introduce significant data disturbance conditions for such retrievals, but they still have the constraints of coincident interjected magnetic fields memory cell state selection scheme for data storage. Thus, there is a desire for a memory cell configuration and corresponding memory cell selection arrangement that allows providing magnetoresistive based memories that are more easily manufacturable but still having the memory cells therein provided with a large areal density.